Boron phosphide-based semiconductor light-emitting device, production method thereof and light-emitting diode

ABSTRACT

A boron phosphide-based semiconductor light-emitting device, which device includes a light-emitting member having a hetero-junction structure in which an n-type lower cladding layer formed of an n-type compound semiconductor, an n-type light-emitting layer formed of an n-type Group III nitride semiconductor, and a p-type upper cladding layer provided on the light-emitting layer and formed of a p-type boron phosphide-based semiconductor are sequentially provided on a surface of a conductive or high-resistive single-crystal substrate and which device includes a p-type Ohmic electrode provided so as to achieve contact with the p-type upper cladding layer, characterized in that a amorphous layer formed of boron phosphide-based semiconductor is disposed between the p-type upper cladding layer and the n-type light-emitting layer. This boron phosphide-based semiconductor light-emitting device exhibits a low forward voltage or threshold value and has excellent reverse breakdown voltage characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is an application filed under 35 U.S.C. §111(a)claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date ofProvisional Applications No. 60/428,716 filed Nov. 25, 2002, No.60/436,640 filed Dec. 30, 2002 and No. 60/436,641 filed Dec. 30, 2002,pursuant to 35 U.S.C. §111(b).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a boron phosphide-based semiconductorlight-emitting device and to a method for producing the same. Moreparticularly, the invention relates to a boron phosphide-basedsemiconductor light-emitting device exhibiting a low forward voltage orthreshold voltage, having excellent reverse breakdown voltagecharacteristics, having a high emission intensity, and having a smalldecrease in emission intensity caused by long-term passage of deviceoperation current. Furthermore, the invention relates to a method forproducing the boron phosphide-based semiconductor light-emitting deviceand to a light-emitting diode comprising the boron phosphide-basedsemiconductor light-emitting device.

2. Description of the Related Art

A Group III nitride semiconductor has been conventionally employed forproducing nitride semiconductor devices such as light-emitting diodes(abbreviated as LEDs) and laser diodes (abbreviated as LDs). See, forexample, Isamu Akasaki, “Group III Nitride Semiconductor,” Dec. 8, 1999,first edition, Baifukan Co., Ltd., Chapters 13 and 14 (hereinafterreferred to as “Non-Patent Document 1”). FIG. 1 shows a cross-section ofa conventional and typical compound semiconductor LED fabricated from astacked structure in which Group III nitride semiconductor layers arestacked on a crystalline substrate. Current Group III nitridesemiconductor LEDs exclusively employ, as a substrate 101, sapphire(α-Al₂O₃ single crystal) or a silicon carbide (chemical formula: SiC)single crystal. On a surface of the substrate 101 is provided a lowercladding layer 102 for attaining “confinement” of light and carriers ina light-emitting layer 103. The lower cladding layer 102 is generallyformed of a Group III nitride having a band gap wider than that of amaterial for forming the light-emitting layer 103; e.g., n-type aluminumgallium nitride (chemical formula: Al_(x)Ga_(1-x)N: 0≦x≦1). See, forexample, the aforementioned Non-Patent Document 1. The light-emittinglayer 103 is stacked on the lower cladding layer 102. The light-emittinglayer 103 is formed of a Group III nitride semiconductor layer havingcompositional proportions of component elements regulated so as toobtain a desired emission wavelength. For example, n-type gallium indiumnitride having an appropriate indium (symbol of element: In) composition(chemical formula: Ga_(x)In_(1-x)N: 0≦x≦1) is generally employed forproducing the light-emitting layer 103. See, for example, JapanesePatent Publication No. 55-3834. On the light-emitting layer 103, anupper cladding layer 106 formed of a Group III nitride semiconductorhaving a conduction type opposite to that of the lower cladding layer102 is provided for exerting a “confinement” effect.

The light-emitting layer 103 is known to employ a quantum well structurefor attaining light emission with a narrow half width in an emissionspectrum and excellent monochromaticity. See, for example, JapanesePatent Application Laid-Open (kokai) No. 2000-133884. In a quantum wellstructure, a well layer 103 a is generally formed of n-typeGa_(x)In_(1-x)N (0≦x≦1). Meanwhile, a barrier layer 103 b, which isprovided so as to attain “confinement” of light and carriers in the welllayer 103 a and attain joining to the well layer 103 a, is formed of aGroup III nitride semiconductor having a band gap wider than that of thewell layer 103 a. For example, the barrier layer is preferably formed ofAl_(x)Ga_(1-x)N (0≦x≦1). See Japanese Patent Application Laid-Open(kokai) No. 2000-133884. The quantum well structure forming thelight-emitting layer 103 is known to be classified into two types; i.e.,a single quantum well (abbreviated as SQW) structure containing only asingle well layer 103 a, and a multiple quantum well (abbreviated asMQW) structure having a plurality of well layers 103 a produced byperiodically and repeatedly stacking joined pairs each consisting of onewell layer 103 a and one barrier layer 103 b. In this connection, thelight-emitting layer 103 shown in FIG. 1 has an MQW structure producedby repeatedly stacking three joined pairs each consisting of one welllayer 103 a and one barrier layer 103 b.

As mentioned above, in a stacked structure 11 for producing conventionalLEDs, an n-type conduction layer (specifically the lower cladding layer102) is disposed on the substrate 101 side, and a p-type conductionlayer serving as the upper cladding layer 106 is disposed on the surfaceside. Thus, the stacked structure is called a p-side-up structure. Thep-side-up LED 10, which is a most typical example of Group III nitridesemiconductor LEDs, is fabricated by forming a p-type Ohmic electrode107 directly on the surface of the p-type upper cladding layer 106 so asto attain contact thereof with the layer. In order to form a p-typeOhmic electrode 107 having a low contact resistance, the p-type uppercladding layer 106 must be formed of a p-type conduction layer havinghigh conductivity. The p-type upper cladding layer 106 has generallybeen conventionally formed of a GaN layer doped with magnesium (symbolof element: Mg). See the aforementioned Non-Patent Document 1. TheMg-doped GaN layer formed through vapor phase growth means has, however,high resistance in an as-grown state. Therefore, the vapor-phase-grownGaN layer must undergo cumbersome treatment such as annealing, orelectron beam treatment in a vacuum, in order to form a p-type layer.See, for example, Japanese Patent Application Laid-Open (kokai) No.53-20882, and Isamu Akasaki, “Group III-V Compound Semiconductor,” May20, 1994, first edition, Baifukan Co., Ltd., Chapter 13. There is alsodisclosed a technique wherein a gallium arsenide nitride (chemicalformula: GaAsN) mixed-crystal layer having a narrow band gap width isprovided on the surface of the upper cladding layer 106 and an Ohmicelectrode is provided so as to attain contact with the mixed-crystallayer. See, for example, Japanese Patent Application Laid-Open (kokai)No. 11-40890.

Boron monophosphide (chemical formula: BP) is known to be a type ofGroup III-V compound semiconductor. See P. Popper et al., “Boronphosphide, a III-V Compound of Zinc-Blende Structure,” (United Kingdom),Nature, May 25, 1957, No. 4569, p. 1075. Boron phosphide is anindirect-transition-type semiconductor exhibiting a relatively lowefficiency of radiation recombination, which provides light emission.See K. Seeger (translated by Keiichi Yamamoto et al.), “Physics Library61, Physics of Semiconductors (the second vol.),” 1st issue, publishedby Yoshioka Shoten, Jun. 25, 1991, p. 507. Therefore, a boron phosphidecrystal layer has been conventionally employed not as an active layerbut as another functional layer included in a semiconductorlight-emitting device or a photo-detector. Specifically, a boronphosphide crystal layer having an n-conduction type (n-type boronphosphide crystal layer) has been employed as an element such as ann-type emitter layer of a hetero-bipolar transistor (HBT) or a windowlayer provided in a pn-junction silicon (Si) solar cell for transmittingsunlight. See Takao Takenaka et al., “Diffusion Layers Formed in SiSubstrates during the Epitaxial Growth of BP and Application toDevices,” (US), Journal of Electrochemical Society, April, 1978, Vol.125, No. 4, p. 633-637.

A p-type crystalline layer can be produced by doping a monomeric boronphosphide (chemical formula: BP)—a type of Group III-V compoundsemiconductor—with magnesium (Mg). See, for example, Japanese PatentApplication Laid-Open (kokai) No. 2-288388. When a light-emitting deviceis fabricated from a p-type boron phosphide crystalline layer, thep-type Ohmic electrode is formed of a gold-zinc (Au-Zn) alloy. See, forexample, Japanese Patent Application Laid-Open (kokai) No. 10-242569.According to a conventional technique, a p-type boron phosphidecrystalline layer is formed through, for example, a metal-organicchemical vapor deposition method (MOCVD) means at a high temperature of850° C. to 1,150° C. See, for example, Japanese Patent ApplicationLaid-Open (kokai) No. 2-288388. Meanwhile, a practically employedtemperature upon vapor phase growth of n-type Ga_(x)In_(1-x)N (0≦x≦1)serving as a well layer included in the aforementioned quantum wellstructure is as low as 600° C. to 850° C. See, for example, JapanesePatent Application Laid-Open (kokai) No. 6-260680. Such low temperatureis employed because vaporization of indium (In) from n-typeGa_(x)In_(1-x)N (0≦x≦1) serving as a well layer, which is extremelythin, is prevented at such a temperature, thereby successfully providinga well layer having an aimed for indium composition.

Meanwhile, boron monophosphide tends to form a p-type conductive layerrather than an n-type conductive layer, because the effective mass of ahole is smaller than that of an electron. See Japanese PatentApplication Laid-Open (kokai) No. 2-288388. In contrast to this, a GroupIII nitride semiconductor such as Al_(x)Ga_(y)In_(z)N (0≦x, y, z≦1,x+y+z=1) tends to readily form an n-type conductive layer and encountersdifficulty in forming a low-resistive p-type conductive layer in anas-grown state.

A conventional technique encounters difficulty in satisfactory formationof a low-resistive p-type boron phosphide crystal layer on an underlyinglayer such as a Group III nitride semiconductor layer. One known methodfor forming a boron phosphide crystal layer having a p-conduction type(p-type boron phosphide crystal layer) is the hydride VPE method, whichemploys sources such as diborane (molecular formula: B₂H₆) and phosphine(molecular formula: PH₃). See Katsufusa Shohno, “SemiconductorTechniques (the first vol.),” 9th issue, University of Tokyo Press, Jun.25, 1992, p. 76-77. In the hydride method, a p-type boron phosphidecrystal layer can be formed through feeding of a boron source and aphosphorus source to a vapor phase growth zone by controlling aconcentration ratio of a phosphorus source to a boron source; i.e., aV/III ratio, to a low ratio. See ibid. However, as the V/III ratio mustbe controlled to a low level as required by the method, ahigh-resistance poly-crystal such as B_(n)P (7≦n≦10), which does notexhibit a semiconductor property, is formed, thereby causing difficultyin successful formation of a low-resistive p-type boron phosphidecrystal layer. See ibid.

When such a conventional technique is employed, successful formation ofa low-resistive p-type boron phosphide crystal layer, without beingaffected by the identity of underlying layer, tends to be difficult. Inaddition, in most cases, conventional techniques have employed a silicon(Si) single crystal as an underlying layer on which a p-type boronphosphide crystal layer is formed (see ibid). Heretofore, no techniquehas been reported for successfully forming a low-resistive p-type boronphosphide crystal layer on a crystalline underlying layer other than asilicon single crystal; e.g., an n-type Group III nitride semiconductorlayer.

SUMMARY OF THE INVENTION

In order to produce a light-emitting device operable at a loweredforward voltage (Vf) or a reduced threshold voltage (Vth), a techniquefor forming a low contact resistance Ohmic electrode is also a criticalissue. With regard to a p-side-up type light-emitting device, aparticularly essential issue is the method of forming a p-type Ohmicelectrode so as to attain contact with a low-resistive p-type conductionlayer. Instead of using a conventional Group III nitride semiconductorlayer which requires cumbersome operations so as to form a low-resistivep-type conduction layer, one conceivable measure is formation of ap-type upper cladding layer from the aforementioned magnesium (Mg)-dopedp-type boron phosphide semiconductor crystalline layer. For example,there can be employed a technique including forming a magnesium(Mg)-doped p-type boron phosphide crystalline layer serving as an uppercladding layer on a light-emitting layer included in a quantum wellstructure formed of stacked thin layers and forming a p-type Ohmicelectrode so as to attain contact with the upper cladding layer, therebyproducing a p-side-up type light-emitting device.

However, as in the aforementioned conventional technique, the suitablevapor phase growth temperature for forming a well layer included in thelight-emitting layer having a quantum well structure greatly differsfrom that for forming a p-type boron phosphide layer. Therefore, whenthe p-type boron phosphide layer is vapor-phase grown at hightemperature, variation in indium content in an indium-containing nitridesemiconductor layer serving as the well layer is induced. The variationin indium content means generally a decrease in indium content, whichreduces stability of quantum levels in the well layer. The decrease instability disturbs successful production of a boron phosphide-basedsemiconductor light-emitting device for emitting light of excellentmonochromaticity having a narrow half width in terms of a desiredwavelength. In addition, a junction barrier difference between a welllayer and a barrier layer (e.g., a GaN barrier layer) decreases, therebyfailing to attain a satisfactory “confinement” effect of light andcarriers, thereby inhibiting provision of a boron phosphide-basedsemiconductor light-emitting device emitting high-intensity light.

At present, even though a readily formable p-type boron phosphidesemiconductor layer is simply provided on a light-emitting layer havinga quantum well structure including Group III nitride semiconductor thinlayers, a boron phosphide-based semiconductor light-emitting devicehaving excellent electrical and emission characteristics cannot besuccessfully produced. This is attributed to the fact that a decisivetechnique has yet to be established for suitably forming a pn-junctionbetween a p-type boron phosphide semiconductor layer and an n-typelight-emitting layer included in a quantum well structure formed of aGroup III nitride semiconductor. In particular, as a barrier layer and awell layer included in a quantum well structure are thin layers having athickness of about some tens of nm or some nm, a technique for joining ap-type boron phosphide semiconductor layer serving as an upper claddinglayer to such thin film without thermally deteriorating the thin layersis required.

In order to solve the above-mentioned problem, the present invention isdirected to the following.

(1) A boron phosphide-based semiconductor light-emitting device, whichdevice includes a light-emitting member having a hetero-junctionstructure in which an n-type lower cladding layer formed of an n-typecompound semiconductor, an n-type light-emitting layer formed of ann-type Group III nitride semiconductor, and a p-type upper claddinglayer provided on the light-emitting layer and formed of a p-type boronphosphide-based semiconductor are sequentially provided on a surface ofa conductive or high-resistive single-crystal substrate and which deviceincludes a p-type electrode provided so as to achieve contact with thep-type upper cladding layer, characterized in that a amorphous layerformed of boron phosphide-based semiconductor is disposed between thep-type upper cladding layer and the n-type light-emitting layer.

(2) A boron phosphide-based semiconductor light-emitting device asdescribed in (1), wherein the amorphous layer has a multilayer structurecomprising a first amorphous layer being in contact with thelight-emitting layer and a second amorphous layer being in contact withthe p-type upper cladding layer and having a carrier concentrationhigher than that of the first amorphous layer.

(3) A boron phosphide-based semiconductor light-emitting device asdescribed in (2), wherein the first amorphous layer is formed of a boronphosphide-based semiconductor grown at a temperature lower than thetemperature at which the light-emitting layer is formed.

(4) A boron phosphide-based semiconductor light-emitting device asdescribed in (2) or (3), wherein the first amorphous layer is formed ofan undoped boron phosphide and has a thickness of 2 nm to 50 nm.

(5) A boron phosphide-based semiconductor light-emitting device asdescribed in (2), wherein the second amorphous layer is formed of ap-type boron phosphide-based semiconductor grown at a temperature higherthan the temperature at which the first amorphous layer is formed.

(6) A boron phosphide-based semiconductor emitting device as describedin (2), wherein the second amorphous layer is formed of an undopedamorphous p-type boron phosphide having an acceptor concentration atroom temperature of 2×10¹⁹ cm⁻³ to 4×10²⁰ cm⁻³, a carrier concentrationat room temperature of 5×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³, and a thickness of 2nm to 450 nm.

(7) A boron phosphide-based semiconductor light-emitting device asdescribed in (1), wherein the p-type upper cladding layer is formed of ap-type boron phosphide-based semiconductor having a dislocation densityequal to or less than that of the Group III nitride semiconductorserving as the light-emitting layer.

(8) A boron phosphide-based semiconductor light-emitting device asdescribed in (1), wherein the p-type upper cladding layer is formed ofan undoped polycrystalline p-type boron phosphide having an acceptorconcentration at room temperature of 2×10¹⁹ cm⁻³ to 4×10²⁰ cm⁻³, acarrier concentration at room temperature of 5×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³,and a resistivity at room temperature of 0.1 Ω·cm or less.

(9) A boron phosphide-based semiconductor light-emitting device asdescribed in (1), wherein the p-type electrode provided on the p-typeupper cladding layer is formed of a bottom-side electrode and a p-typeOhmic electrode; the bottom-side electrode in contact with the surfaceof the p-type upper cladding layer and being formed of a material ableto form non-Ohmic contact with the p-type boron phosphide-basedsemiconductor serving as the p-type upper cladding layer; and the p-typeOhmic electrode being in electrical contact with the bottom-sideelectrode, extending so as to achieve contact also with the surface ofthe p-type upper cladding layer, and being in Ohmic contact with thep-type boron phosphide-based semiconductor.

(10) A boron phosphide-based semiconductor light-emitting device asdescribed in (9), wherein the p-type Ohmic electrode is provided so asto extend, as a stripe electrode, on a portion of the surface of thep-type upper cladding layer where the bottom-side electrode is notprovided.

(11) A boron phosphide-based semiconductor light-emitting device asdescribed in (9), wherein the bottom-side electrode is formed of agold-tin (Au—Sn) alloy or a gold-silicon (Au—Si) alloy.

(12) A boron phosphide-based semiconductor light-emitting device asdescribed in (9), wherein the bottom-side electrode is formed oftitanium (Ti).

(13) A boron phosphide-based semiconductor light-emitting device asdescribed in (9), wherein the p-type Ohmic electrode is formed of agold-beryllium (Au—Be) alloy or a gold-zinc (Au—Zn) alloy.

(14) A boron phosphide-based semiconductor light-emitting device asdescribed in (9), wherein the p-type Ohmic electrode is formed of nickel(Ni) or a compound thereof.

(15) A boron phosphide-based semiconductor light-emitting device asdescribed in (9), wherein an intermediate layer formed of a transitionmetal is provided between the p-type Ohmic electrode and the bottom-sideelectrode.

(16) A boron phosphide-based semiconductor light-emitting device asdescribed in (15), wherein the intermediate layer is formed ofmolybdenum (Mo) or platinum (Pt).

(17) A method for producing a boron phosphide-based semiconductorlight-emitting device, the method including forming a light-emittingmember having a hetero-junction structure in which an n-type lowercladding layer composed of an n-type compound semiconductor, an n-typelight-emitting layer composed of an n-type Group III nitridesemiconductor, and a p-type upper cladding layer composed of a p-typeboron phosphide-based semiconductor and provided on the light-emittinglayer are sequentially provided on a surface of a conductive orhigh-resistive single-crystal substrate, and forming a p-type Ohmicelectrode so as to achieve contact with the p-type upper cladding layer,characterized in that the method comprises forming an amorphous layercomposed of a boron phosphide-based semiconductor on the n-typelight-emitting layer through a vapor phase growth method, and formingthe p-type upper cladding layer composed of a p-type boronphosphide-based semiconductor layer on the amorphous layer through avapor phase growth method.

(18) A method for producing a boron phosphide-based semiconductorlight-emitting device, the method including forming a light-emittingmember having a hetero-junction structure in which an n-type lowercladding layer composed of an n-type compound semiconductor, an n-typelight-emitting layer composed of an n-type Group III nitridesemiconductor, and a p-type upper cladding layer composed of a p-typeboron phosphide-based semiconductor and provided on the light-emittinglayer are sequentially provided on a surface of a conductive orhigh-resistive single-crystal substrate, and forming a p-type Ohmicelectrode so as to achieve contact with the p-type upper cladding layer,characterized in that the method comprises forming a first amorphouslayer composed of boron phosphide-based semiconductor on the n-typelight-emitting layer through a vapor phase growth method; forming asecond amorphous layer composed of amorphous p-type boronphosphide-based semiconductor having a carrier concentration higher thanthat of the first amorphous layer through a vapor phase growth methodsuch that the second amorphous layer is joined to the first amorphouslayer; and forming the p-type upper cladding layer composed of a p-typeboron phosphide-based semiconductor layer through a vapor phase growthmethod such that the upper cladding layer is joined to the secondamorphous layer.

(19) A method for producing a boron phosphide-based semiconductorlight-emitting device as described in (18), wherein the first amorphouslayer is formed on the n-type light-emitting layer maintained at atemperature higher than 250° C. and lower than 750° C. through a vaporphase growth method at a concentration ratio of a boron-containingcompound as a boron source to a phosphorus-containing compound as aphosphorus source fed to a vapor phase growth zone (V/III ratio) fallingwithin a range of 0.2 to 50.

(20) A method for producing a boron phosphide-based semiconductorlight-emitting device as described in (18), wherein the second amorphouslayer is vapor-phase grown on the first amorphous layer maintained at atemperature of 1000° C. to 1250° C. at a V/III ratio higher than thatemployed in vapor phase growth of the first amorphous layer.

(21) A method for producing a boron phosphide-based semiconductorlight-emitting device as described in (17) or (18), wherein the p-typeupper cladding layer is vapor-phase grown at a temperature of 750° C. to1200° C. at a V/III ratio falling within a range of 600 to 2,000.

(22) A method for producing a boron phosphide-based semiconductorlight-emitting device as described in (18), wherein each of the firstamorphous layer, the second amorphous layer, and the p-type uppercladding layer is composed of boron phosphide (BP).

(23) A light-emitting diode comprising a boron phosphide-basedsemiconductor light-emitting device as described in any one of (1) to(16).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a conventional LED.

FIG. 2 is a schematic cross-sectional view of an LED described inExample 1.

FIG. 3 is a schematic cross-sectional view of an LED described inExample 2.

FIG. 4 is a schematic plane view of the LED shown in FIG. 3.

FIG. 5 is a schematic cross-sectional view of an LED described inExample 3.

FIG. 6 is a schematic plane view of the LED shown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

The stacked structure for producing the boron phosphide-basedsemiconductor light-emitting device according to the present inventionis formed such that silicon (Si) single crystal, a Group III-V compoundsemiconductor single crystal (e.g., gallium nitride (GaN), or galliumphosphide (GaP)) or an oxide single crystal (e.g., sapphire (Al₂O₃single crystal)) serves as a substrate. When a p-side-up stackedstructure is provided on a conductive single-crystal substrate, ann-type conductive substrate is preferably employed. For example, aphosphorus (P)-doped n-type silicon single-crystal substrate can beemployed.

An n-type lower cladding layer is provided on a surface of thesingle-crystal substrate. The n-type lower cladding layer is deposited,for example, through vapor phase growth means such as the metal-organicchemical vapor deposition method (MOCVD). In a p-side-up light-emittingdevice, a lower cladding layer provided between the single-crystalsubstrate and the light-emitting layer is preferably formed of an n-typelow-resistive conductive layer having a resistivity (specificresistance) at room temperature of 1 Ω·cm or less. The n-type lowercladding layer is formed of an n-type compound semiconductor, and isformed of, for example, a Group III-V compound semiconductor such as ann-type gallium nitride. In particular, a low-resistive n-type boronphosphide having a resistivity less than 0.1 Ω·cm is preferably used toform an n-type lower cladding layer.

A light-emitting layer is provided on the lower cladding layer. Thelight-emitting layer is formed of an n-type Group III nitridesemiconductor. The light-emitting layer can be stacked through vaporphase growth means. The light-emitting layer is formed from asemiconductor material having a forbidden band gap corresponding to adesired emission wavelength. For example, a blue-light-emitting layermay be formed of direct-transition-type gallium indium nitride(compositional formula: Ga_(x)In_(1-x)N: 0<x<1) or gallium nitridephosphide (compositional formula: GaN_(y)P_(1-y): 0<y<1). Throughappropriate selection of compositional proportions of indium (In) andphosphorus (P) (i.e., 1-x and 1-y), a light-emitting layer for emittinglight of a wavelength falling within a near ultraviolet region or agreen light region can also be provided. For example, by use ofwurtzite-type gallium nitride mixed crystals, an n-type conductive layercan be formed more readily than a p-type conductive layer, in accordancewith the degenerated structure of the valence band of the semiconductormaterial. Thus, a Group III nitride semiconductor such as n-typeGa_(x)In_(1-x)N (0≦x≦1) can be utilized as a material to form an n-typelight-emitting layer.

The light-emitting layer has preferably a quantum well structure havingone or more well layers and one or more barrier layers. The quantum wellstructure enables light emission with a narrow half width in an emissionspectrum and excellent monochromaticity. The quantum well structure maybe a single quantum well (SQW) structure containing only a single welllayer, or a multiple quantum well (MQW) structure having a plurality ofwell layers produced by periodically and repeatedly stacking joinedcouples each consisting of one well layer and one barrier layer.

A p-type upper cladding layer having a conduction type opposite that ofan n-type lower cladding layer is provided on the n-type light-emittinglayer. The p-type upper cladding layer is formed from a p-type boronphosphide-based semiconductor instead of a Group III nitridesemiconductor, which encounters difficulty in readily providing alow-resistive p-type conductive layer, because of the aforementionedvalence band structure. The p-type upper cladding layer is preferablyformed of boron monophosphide (BP). Particularly, the upper claddinglayer is formed of polycrystalline boron phosphide.

According to the present invention, an upper cladding layer formed of ap-type boron phosphide-based semiconductor is formed on thelight-emitting layer, with an amorphous layer formed of a boronphosphide-based semiconductor intervening between the two layers.

The boron phosphide-based semiconductor refers to a cubiczincblende-type Group III-V compound semiconductor containing boron(symbol of element: B) and phosphorus (symbol of element: P). Specificexamples include B_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)As_(δ) (0<α≦1,0≦β<1, 0≦γ<1, 0<α+β+γ≦1, 0≦δ<1) andB_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)N_(δ) (0<α≦1, 0≦β<1, 0≦γ<1,0<α+β+γ≦1, 0≦δ<1). More specifically, the semiconductor is monomericboron phosphide (BP), boron gallium indium phosphide (compositionalformula: B_(α)Ga_(γ)In_(1-α-γ)P: 0<α≦1, 0≦γ<1), or a mixed-crystalcompound containing a plurality of Group V element species such as boronnitride phosphide (compositional formula: BP_(1-δ)N_(δ): 0≦δ<1) or boronarsenide phosphide (compositional formula: B_(α)P_(1-δ)As_(δ)). Inparticular, monomeric boron phosphide (BP) is an essential element ofboron phosphide-based semiconductor mixed-crystals, and boronphosphide-based mixed-crystals having a wide band gap can be formed froma BP having a wide band gap serving as a basic constituent.

The amorphous layer formed of boron phosphide can be vapor-phase grownthrough the halogen method (see “Journal of the Japanese Association forCrystal Growth,” Vol. 24, No. 2, (1997), p. 150) by use of sources suchas boron trichloride (molecular formula: BCl₃) and phosphorustrichloride (molecular formula: PCl₃). Alternatively, vapor phase growthcan also be performed through the hydride method (see J. Crystal Growth,24/25 (1974), p. 193-196) by use of sources such as borane (molecularformula: BH₃) or diborane (molecular formula: B₂H₆), and phosphine(molecular formula: PH₃), or through molecular beam epitaxy (see J.Solid State Chem., 133 (1997), p. 269-272). Metal-organic chemical vapordeposition (MOCVD) (see Inst. Phys. Conf. Ser., No. 129 (IOP PublishingLtd. (UK, 1993), p. 157-162) employing an organic boron compound and aphosphorus hydride as sources can also be employed.

MOCVD is a particularly advantageous growth method for growing theamorphous layers at low temperature, since a highly decomposablesubstance such as triethylborane (molecular formula: (C₂H₅)₃B) isemployed as a boron source. Specifically, the growth is performedthrough atmospheric pressure (near atmospheric pressure) orreduced-pressure MOCVD employing a triethylborane/phosphine (molecularformula: PH₃)/hydrogen (H₂) reaction system at 250° C. to 1,200° C. Whenthe temperature is higher than 1,200° C., poly boron phosphide crystalssuch as B₁₃P₂ tend to be formed (see J. Am. Ceramic Soc., 47(1) (1964),p. 44-46), thereby failing to attain successful formation of anamorphous layer formed of monomeric boron phosphide. When thetemperature is higher than 750° C., a polycrystalline layer containingboron and phosphorus tends to be readily formed. In suchhigh-temperature growth, an amorphous layer can be formed by reducing asupply ratio of the amount of phosphorus to the amount of boron; i.e.,V/III, to a low level. Specifically, when the V/III ratio (=(C₂H₅)₃B/PH₃supply concentration ratio) is controlled to a low level falling withina range of 0.2 to 50 in MOCVD employing the above reaction system, anamorphous layer can be successfully formed at comparatively hightemperature.

When the light-emitting layer has a quantum well structure, theamorphous layer formed of a boron phosphide-based semiconductor can beprovided on any of the barrier layer or the well layer serving as theuppermost surface end (final end) of the quantum well structure (i.e.,the light-emitting layer). Most preferably, there is employed a stackedstructure in which the amorphous layer is provided so as to attainjoining to the barrier layer serving as the final end. The barrier layerserving as the final end, when provided so as to be joined to the welllayer, serves as a coating layer for the well layer and effectivelyprevents loss of the well layer caused by, for example, sublimationduring vapor phase growth of an amorphous boron phosphide-basedsemiconductor layer. Through employment of the technique for providingan amorphous boron phosphide-based semiconductor layer at a temperatureequal to or lower than the vapor phase growth temperature at which aGroup III nitride semiconductor layer serving as a barrier layer or awell layer is formed, thermal deterioration of the well layer caused bycondensation of elements such as indium (In) can be prevented.Particularly when the vapor phase growth is performed at a temperaturelower than the well layer growth temperature, thermal deterioration ofthe barrier layer as well as the well layer can be effectively avoided.However, as mentioned above, a temperature lower than 250° C. is notsuited for forming an amorphous boron phosphide-based semiconductorlayer, from the viewpoint of poor thermal decomposition efficiency ofelement sources, although the amorphous boron phosphide-basedsemiconductor layer is preferably vapor-phase grown at a temperaturelower than the barrier layer growth temperature and well layer growthtemperature.

Notably, the thickness of amorphous layer can be directly determinedthrough, for example, observation under a transmission electronmicroscope (TEM). Whether or not the formed layer is amorphous can bedetermined on the basis of electron-beam diffraction patterns or X-raydiffraction patterns. An amorphous layer will exhibit a halo electrondiffraction pattern. The stoichiometric ratio of phosphorus to boron,the two elements forming the amorphous layers, is determined fromquantitative elemental measurements of boron and phosphorus on the basisof, for example, Auger electron spectroscopy.

The amorphous layer may have a single layer structure or a multilayerstructure having two or more layers. When the amorphous layer has amultilayer structure, an amorphous layer which is in contact with thelight emitting layer is, hereinafter, referred to as a first amorphouslayer, and an amorphous layer which is in contact with the uppercladding layer is referred to as a second amorphous layer.

In order to form the first amorphous layer which is tightly anduniformly joined to the light-emitting layer, the V/III ratio ispreferably controlled to a relatively high level falling within theaforementioned range. For example, the amorphous layer is formed at aV/III ratio of 45. The amorphous boron phosphide layer formed at acomparatively high V/III ratio assumes a high-resistance layer having acarrier (hole) concentration of 5×10¹⁷ cm⁻³ or less. In other words, thefirst amorphous layer tightly joined to the light-emitting layer issuitably formed of a high-resistance layer containing boron andphosphorus at a stoichiometric composition.

The first amorphous layer provides “adsorption sites” for growing thesecond amorphous layer, thereby promoting uniform vapor phase growth. Inorder to promote uniform growth of the second amorphous layer, the firstamorphous layer preferably has a thickness of about 1 nm or more, morepreferably of 20 nm to 50 nm, which is sufficient to uniformly andsufficiently cover the surface of the light-emitting layer. From anotheraspect, the first amorphous layer preferably has a thickness of 50 nm orless so as to successfully permit passage of operation current fordriving a light-emitting device, as the first amorphous layer has arelatively high resistance as mentioned above. Furthermore, the firstamorphous layer preferably has a thickness of 5 nm to 20 nm. Thethickness of the first amorphous layer is controlled by regulating thetime of feeding a boron source to a growth zone.

When an additional second amorphous layer formed of a boronphosphide-based semiconductor is provided on the aforementioned firstamorphous layer through vapor phase growth at a temperature higher thanthe first amorphous layer growth temperature, a low-resistive p-typeboron phosphide-based semiconductor single-crystal layer can be readilyformed on the second amorphous layer in an as grown state. The secondamorphous layer contributing to provide a low-resistive p-type boronphosphide-based semiconductor single-crystal layer at room temperatureis preferably formed of a boron phosphide-based semiconductor layerwhich is stoichiometrically rich in a Group III element such as boronwith respect to a Group V element such as phosphorus. The secondamorphous layer which is stoichiometrically rich in boron can be formedsuitably at a temperature higher than the lower first amorphous layergrowth temperature. According to one specific method, a first amorphouslayer is formed on the light-emitting layer at 350° C. to 650° C. and,subsequently, a second amorphous layer is formed at 1,000° C. to 1,200°C., to thereby provide an amorphous layer serving as an underlying layerfor forming a low-resistive p-type boron phosphide-based semiconductorsingle-crystal layer in an as-grown state. Although there is a techniquein which two amorphous layers are formed by changing the vapor phasegrowth means, a more readily practiced and convenient technique includesformation of a first amorphous layer on the light-emitting layer,followed by formation of a second amorphous layer serving as anunderlying layer for producing a p-type boron phosphide-basedsemiconductor single-crystal layer. When the second amorphous layerserving as an underlying layer for producing a p-type boronphosphide-based semiconductor single-crystal layer is formed at arelatively high temperature, the first amorphous layer provided so as toattain joining to the light-emitting layer serves as a protective layerwhich can prevent thermal decomposition of the light-emitting layer.

A second amorphous layer formed of a p-type boron phosphide-basedsemiconductor is stacked on the first amorphous layer. The secondamorphous layer, which is tightly joined to the light-emitting layer byvirtue of the action of the first amorphous layer, is effective forproviding a p-type boron phosphide crystalline layer. The secondamorphous layer formed of boron phosphide can also be formed through theaforementioned vapor phase growth method. In order to effectivelyprovide a p-type upper cladding layer, the second amorphous layer ispreferably formed of a p-type amorphous boron phosphide conductive layerwhich is stoichiometrically rich in boron with respect to phosphorus.The amorphous boron phosphide layer stoichiometrically rich in boron canbe formed by controlling the V/III ratio to a lower level during vaporphase growth. With an increase in amount of boron in the above case, thecarrier (hole) concentration increases. The second amorphous layerpreferably has a carrier concentration of 5×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³. Inaccordance with the carrier concentration, the acceptor concentration atroom temperature is preferably 2×10¹⁹ cm⁻³ to 4×10²⁰ cm⁻³. When the holeconcentration of the second amorphous layer falls below theaforementioned range, caused by electrical compensation of an acceptorcomponent by a donor component present in an excessive amount, thesecond amorphous layer generally exhibits high resistance. Specifically,such an amorphous layer encounters difficulty in production of an LEDproviding a low forward voltage (Vf). In contrast to this, when the holeconcentration is higher than the aforementioned range, an acceptorcomponent contained in an excessive amount in the second amorphous layerdiffuses and migrates into the light-emitting layer, to therebyelectrically compensate for n-type carriers (electrons) contained in thelight-emitting layer. In this case, the resistance of the light-emittinglayer increases, which is disadvantageous.

Each of the first and the second amorphous layers is preferably formedof an undoped boron phosphide layer without intentionally doping with animpurity (i.e., without doping). The reason for employing the aboveprocess is to prevent variation in resistance of the light-emittinglayer caused by diffusion of the impurity present in the amorphouslayers to the light-emitting layer occurring during vapor phase growthof the amorphous boron phosphide layers. The second amorphous layer,which serves as an underlying layer on which a p-type polycrystallineboron phosphide conductive layer serving as a p-type upper claddinglayer is formed, preferably has a thickness of 2 nm to 450 nm. When thelayer is extremely thin; i.e., has a thickness less than 2 nm, thesurface of the first amorphous layer is not completely, sufficiently,and homogeneously covered, thereby failing to produce a p-type uppercladding layer having excellent in-plane homogeneity in terms ofthickness and carrier concentration. A thickness, of the boron-richamorphous layer, in excess of 450 nm is disadvantageous for attaining aflat surface of the amorphous layer.

The suitable layer formation temperature of the first amorphous layerdiffers from that of the second amorphous layer, because the two layersserve different functions. Hereinafter, the formation steps for eachlayer will be described in detail.

The first amorphous layer is formed directly on the surface of theunderlying crystal in order to relax the lattice mismatch between theunderlying crystal and the p-type boron phosphide crystal layer. Throughprovision of such a layer, a p-type boron phosphide crystal layer freefrom misfit dislocations with excellent adhesion to the underlyingcrystal can be formed.

The first amorphous layer having the above function can be formedthrough vapor phase growth by feeding a boron-containing compound (boronsource) and a phosphorus-containing compound (phosphorus source) to avapor phase growth zone at a temperature higher than 250° C. and lowerthan 750° C. In order to form the first amorphous layer through thevapor phase growth method at a temperature higher than 250° C. and lowerthan 750° C., the underlying crystal is placed in a vapor phase growthzone and the underlying crystal is heated to a temperature higher than250° C. and lower than 750° C. for vapor phase growth. Notably, when thelayer formation temperature (temperature of underlying crystal) is 250°C. or lower, thermal decomposition of a boron source and a phosphorussource does not proceed sufficiently, possibly failing to form a layercontaining boron and phosphorus, whereas when the layer formationtemperature is 750° C. or higher, the formed layer has a polycrystallineor single-crystal structure, possibly failing to form an amorphouslayer.

The first amorphous layer can be effectively formed when the V/III ratiois controlled to a low level during vapor phase growth. Specifically, inorder to successfully form the first amorphous layer at theaforementioned layer formation temperature, the V/III ratio ispreferably controlled to 0.2 to 50, more preferably 2 to 50. Forexample, when layer formation is performed through a halogen vapor phasegrowth method, and boron tribromide (chemical formula: BBr₃) andphosphorus trichloride (chemical formula: PCl₃) are used as sources, theV/III ratio is preferably controlled to about 10. When a V/III ratio istoo low, a layer containing aggregated spherical shape boron crystalsand having poor surface flatness may be formed, possibly resulting inpoor surface flatness of the p-type boron phosphide crystal layer to beformed in a subsequent step, whereas when the V/III ratio is elevated tohigher than 50, a polycrystalline layer may be formed, thereby possiblyfailing to successfully form the first amorphous layer.

In the present invention, the first amorphous layer containing boronatoms and phosphorus atoms is preferably a p-type conductive layer whichis stoichiometrically rich in boron atoms. The reason is as follows. Asdescribed hereinafter, the second amorphous layer is preferably a p-typeconductive layer, so that a p-type boron phosphide crystal layer issuccessfully formed. The second amorphous layer is grown whileinheriting the nature of the first amorphous layer. Therefore, in orderto yield the second amorphous layer serving as a p-type conductivelayer, the first amorphous is preferably a p-type conductive layer.

The first amorphous layer preferably has a thickness of 2 nm to 50 nm.When the thickness of the first amorphous layer is less than 2 nm, thesurface of the underlying crystal to be covered might fail to be coatedwith the amorphous layer sufficiently and uniformly. As a result,deformation due to a difference in thermal expansion coefficient orother factors is not uniformly relaxed, possibly causing peeling off ofthe p-type boron phosphide crystal layer from the underlying crystal.Meanwhile, the surface of the underlying crystal can be uniformly coatedwith the first amorphous layer, leading to resolution of the aboveproblem, provided that the first amorphous layer has a thickness of 2 nmor more. In addition to this, the first amorphous layer functions as asurface-protection layer for preventing thermal decomposition of theunderlying crystal during the formation of the first amorphous layer.This function is successfully attained by increasing the layer thicknessto 2 nm or more. This is critical, particularly when a Group III nitridesemiconductor or a similar material, which is prone to thermallydecompose due to vaporization of the Group V element or other factors,is employed as an underlying crystal or when the first amorphous layeris formed at a high layer formation temperature. Needless to say, withincreasing thickness of the first amorphous layer; i.e., asurface-protection layer for the underlying crystal, the functionthereof is more effectively attained. A layer thickness in excess of 50nm is not preferred, because single-crystal grains may be formed in thefirst amorphous layer, or a polycrystalline layer may be formed.

The second amorphous layer functions as an underlying layer fordepositing a p-type boron phosphide crystal layer. Through provision ofthe second amorphous layer, the p-type boron phosphide crystal layer canbe formed successfully and readily. In addition, the second amorphouslayer functions as a protection layer for preventing thermaldecomposition of the first amorphous layer during vapor phase growth ofthe second amorphous layer.

Similar to the case of the first amorphous layer, the second amorphouslayer can be formed through vapor phase growth by feeding aboron-containing compound (boron source) and a phosphorus-containingcompound (phosphorus source) to a vapor phase growth zone. The vaporphase growth method for forming the second amorphous layer may be thesame vapor phase growth method as employed in formation of the firstamorphous layer or a method different from the above method. From theviewpoint of production efficiency or other factors, the former methodis preferred. The latter method may be performed by growing a firstamorphous layer through the hydride method employing a diborane(B₂H₆)/phosphine (PH₃)/hydrogen (H₂) system and forming a secondamorphous layer through MOCVD. Any appropriate combination of methodsmay be employed.

In the present invention, the second amorphous layer containing boronatoms and phosphorus atoms preferably has a compositionstoichiometrically rich in boron atoms. Through provision of the secondamorphous layer, a p-type boron phosphide crystal layer can besuccessfully formed thereon. The equivalent stoichiometric compositionof boron phosphide accounts for a ratio of 1:1 (boron atoms:phosphorusatoms). When a boron-rich second amorphous layer is formed from boronphosphide, layer formation is preferably performed such that the numberof boron atoms exceeds about 0.5 to 1.0% the number of phosphorus atoms.

The layer formation temperature at which the second amorphous layer isformed is preferably 1,000° C. to 1,200° C. When such a layer formationtemperature is employed, a second amorphous layer stoichiometricallyrich in boron can be successfully formed. Since the underlying crystalsurface has already been coated with the first amorphous layer whichfunctions as a surface-protection layer, thermal decomposition of theunderlying crystal is prevented even when the second amorphous layer isformed at a temperature of 1,000° C. or higher.

Similar to the case of the first amorphous layer, the V/III ratio atwhich the second amorphous layer is formed preferably falls within arange of 2 to 50. As the preferred layer formation temperature for thesecond amorphous layer is higher than that for the first amorphouslayer, the second amorphous layer is preferably formed at a V/III ratiogreater than that employed in formation of the first amorphous layer. Toperform layer formation at a V/III ratio greater than that employed inthe formation of the first amorphous layer, one possible method is thatthe amount of the phosphorus source (Group V source) is increased whilethe amount of the boron source (Group III source) to be fed to a vaporphase growth zone is maintained at a level similar to that employed information of the first amorphous layer. The second amorphous layer isformed at the thus-increased V/III ratio. Upon formation of the secondamorphous layer, controlling of the V/III ratio to a high level withinthe aforementioned range is preferred, because an excellent surfaceflatness of the formed second amorphous layer can be attained. Inaddition, vaporization of elements such as boron and phosphoruscontained in the first amorphous layer during vapor phase growth of thesecond amorphous layer can also be prevented, provided that the V/IIIratio is controlled to a higher level.

Similar to the case of the first amorphous layer, the second amorphouslayer preferably has a thickness of 2 nm to 50 nm. Particularly when apn-junction structure of a compound semiconductor light-emitting devicehaving a pn-junction-type light-emitting portion is fabricated with ann-type Group III nitride semiconductor as an underlying crystal, thetotal thickness of the first and the second amorphous layers ispreferably controlled to 100 nm or less. The reason for controlling thetotal thickness is as follows. Because at least the second amorphouslayer of the first and the second amorphous layers has a compositionwhich is stoichiometrically rich in boron, flow resistance of deviceoperation current to a light-emitting portion provided by the presenceof the first and the second amorphous layers is reduced to a certainextent. If the first and the second amorphous layers are, for example,p-type layers of π-type and the total layer thickness is in excess of100 nm, flow resistance of device operation current to thelight-emitting portion provided by the presence of the first and thesecond amorphous layers increases.

A p-type upper cladding layer formed of p-type boron phosphide isprovided on the second amorphous layer. The upper cladding layer can beformed through the aforementioned vapor phase growth method for formingthe first and the second amorphous layers. The p-type boron phosphidelayer serving as the upper cladding layer is preferably formed of alow-resistance conductive layer having a low resistivity in order toform a p-type electrode having excellent Ohmic characteristics. Anundoped polycrystalline p-type boron phosphide having a resistivity atroom temperature of 0.1 Ω·cm or less is particularly preferred forforming the p-type boron phosphide layer. The upper cladding layerformed of a p-type conductive layer having such a low resistance can beemployed as a contact layer for forming a p-type Ohmic electrode. Thep-type upper cladding layer having a resistivity of 0.1 Ω·cm or less isessentially formed on an underlying layer; i.e., the second amorphousboron phosphide layer having an acceptor concentration at roomtemperature of 2×10¹⁹ cm⁻³ to 4×10²⁰ cm⁻³ and a carrier concentration atroom temperature of 5×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³. The polycrystallinelayer serving as the p-type upper cladding layer is grown whileinheriting the p-type conductivity of the amorphous boron phosphidelayer serving as the underlying layer.

The stoichiometric conditions of the second amorphous layer; i.e., richin boron and deficient in phosphorus, themselves, are transferred to thepolycrystalline boron phosphide layer serving as the upper claddinglayer. Therefore, electrical properties of the second amorphous layer,themselves, are transferred to the upper cladding layer, whereby theupper cladding layer assumes the form of a p-type conductivepolycrystalline layer. When the aforementioned low-resistivity p-typeboron phosphide serving as the p-type conductive layer is formed at thesame V/III ratio as employed for forming the second amorphous layer, thep-type boron phosphide is advantageously formed at a temperature notlower than the second amorphous layer formation temperature and nothigher than 1,200° C. In addition, decreasing the V/III ratio (alsoemployed for forming the second amorphous layer) to a low level fallingwithin the aforementioned preferred range is advantageous for obtainingthe p-type polycrystalline layer having a low resistivity in an as grownstate. The polycrystalline p-type boron phosphide layer serving as theupper cladding layer preferably has a carrier concentration at roomtemperature of 5×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³. When the carrierconcentration is lower than 5×10¹⁸ cm⁻³, a low-resistance p-typeconductive layer having a resistivity of 0.1 Ω·cm or less cannot beformed, although the mobility at room temperature is enhanced, whereaswhen the carrier concentration is higher than 1×10²⁰ cm⁻³, absorption ofthe light emitted from the light-emitting layer increases, which is notpreferred for producing an LED of high emission intensity. Furthermore,the p-type boron phosphide layer preferably has an acceptorconcentration at room temperature of 2×10¹⁹ cm⁻³ to 4×10²⁰ cm⁻³. A largeacceptor concentration in excess of 4×10²⁰ cm⁻³ is not preferred,because a polycrystalline boron phosphide layer having a surface lackingin flatness is formed, which is detrimental to provision of an Ohmicelectrode mentioned hereinafter.

In a p-side-up light-emitting device, in which the light emitted from alight-emitting layer such as an LED or an surface emission LD is takento the outside in a vertically upward direction, the p-type uppercladding layer is essentially formed of undoped polycrystalline p-typeboron phosphide, which can transmit the light emitted from thelight-emitting layer to the outside at high efficiency. The performanceof transmission of emitted light to the outside (represented bytransmittance) decreases exponentially with increasing thickness of thep-type upper cladding layer. Thus, when the maximum layer thickness ofthe p-type upper cladding layer, having the aforementioned preferredcarrier concentration, is controlled to 5×10⁻⁴ cm (=5 μm) or less, thep-type upper cladding layer having excellent transmittance can beformed. When the p-type upper cladding layer is formed from apolycrystalline layer, strain attributed to lattice mismatch with thelight-emitting layer material is effectively absorbed. As a result,strain to be provided to the light-emitting layer can be reduced,despite use of a thick polycrystalline layer. Therefore, unstablefluctuation in wavelength of the light emitted from the light-emittinglayer, the fluctuation being caused by strain provided to the layer, canbe effectively prevented. On the other hand, the p-type upper claddinglayer included in a plane-type LED is required to supply forward currentfor driving the device to a wide area of the light-emitting layer formedthereunder. In this case, the upper cladding layer preferably has athickness of 50 nm or more.

For example, when the upper cladding layer is formed from apolycrystalline p-type boron phosphide layer having a thickness of 1 μmand a carrier concentration of 2×10¹⁹ cm⁻³, the layer also serves as awindow layer having a transmittance higher than 40% with respect to bluelight (wavelength: 450 nm). A p-type upper cladding layer having ahigher transmittance can be formed from a polycrystalline p-type boronphosphide layer which maintains a resistivity at 0.1 Ω·cm or lower andhas a lower carrier concentration and a smaller thickness. In p-side-upnitride semiconductor LEDs, a p-type Group III nitride semiconductorlayer serving as the upper cladding layer has a high resistivity,thereby failing to satisfactorily diffuse forward current, over theentirety of the light-emitting layer, uniformly. For solving theproblem, in a conventional Group III nitride semiconductor LED, atransparent electrode formed of, for example, nickel (symbol of element:Ni) is generally provided for uniformly diffusing the forward current ona p-type cladding layer. However, the transmittance of emitted lightremains less than 40%, because such a metallic or metal oxide filmabsorbs emitted light. According to the advantageous structure of thepresent invention, a p-type upper cladding layer having excellenttransmittance of emitted light can be provided without intentionallyproviding a transparent electrode for diffusing the forward current, anda light-emitting device can be readily provided.

By the mediation of an amorphous layer formed of a boron phosphide-basedsemiconductor, a p-type upper cladding layer formed of a low-resistancep-type boron phosphide-based semiconductor can be formed. In addition, ap-type upper cladding layer formed of a p-type boron phosphide-basedsemiconductor of high quality having a small dislocation density can beeffectively formed. In a light-emitting layer formed on a crystallinesubstrate having a poor lattice matching degree, dislocationspenetrating the light-emitting layer are generally present at a densityhigher than about 10¹⁰ cm⁻². The amorphous layer according to thepresent invention formed of a boron phosphide-based semiconductor andprovided so as to attain joining to the light-emitting layer can inhibittransfer of such high-density dislocations into the p-type uppercladding layer formed of a p-type boron phosphide-based semiconductor atthe junction interface between the amorphous layer and thelight-emitting layer. Thus, by the mediation of the amorphous layerformed of a boron phosphide-based semiconductor, a p-type upper claddinglayer formed of a low-resistive p-type boron phosphide-basedsemiconductor exhibiting p-type conduction and having excellentcrystallinity with a dislocation density of 1×10³ cm⁻³ or less can beprovided in an as-grown state. Such a low-resistive p-type boronphosphide-based semiconductor layer having a low dislocation density canbe advantageously employed for forming a p-type cladding layer which canprevent breakdown voltage failure caused by local leakage of deviceoperation current occurring via dislocations.

The temperature at which the p-type upper cladding layer formed of ap-type boron phosphide-based semiconductor is formed preferably fallswithin a range of 1,000° C. to 1,200° C. A layer formation temperaturehigher than 1,200° C. is not preferred, because a polyboron species suchas B₁₃P₂ may be formed.

In this step, the V/III ratio is preferably controlled to a greatlyhigher level than that employed in vapor phase growth for forming thefirst or the second amorphous layer, specifically 600 to 2,000.

A p-type boron phosphide crystal layer can be formed on thestoichiometrically boron-rich second amorphous layer withoutintentionally adding an impurity for attaining p-type conduction (p-typedopant) (i.e., without doping). Specifically, a low-resistive p-typeboron phosphide crystal layer having a carrier (hole) concentration ofabout 2×10¹⁹ cm⁻³ and a resistivity of about 5×10⁻² Ω·cm can be formedwithout doping through MOCVD at 1,025° C. by use of a triethylborane((C₂H₅)₃B)/phosphine (PH₃)/hydrogen (H₂) system.

As described above, the present invention has an advantage in that ap-type boron phosphide crystal layer can be readily formed withoutdoping. Alternatively, the p-type boron phosphide crystal layer may alsobe formed by adding a p-type impurity such as silicon (Si). Because thesilicon impurity serves as a remarkably effective p-type impurity whencontained in a boron phosphide crystal layer which is rich in boron withrespect to phosphorus, a low-resistance boron phosphide crystal layer isformed by doping boron phosphide with silicon. Examples of silicondopant sources include silane (molecular formula: SiH₄), disilane(molecular formula: Si₂H₆), and halosilicon compounds such as silicontetrachloride (molecular formula: SiCl₄). A mixed gas such as Si₂H₆-H₂can also be used.

Notably, the silicon impurity serves as an n-type impurity, whencontained in a phosphorus-rich boron phosphide crystal layer, andelectrically compensates for acceptors. Therefore, doping of aphosphorus-rich boron phosphide layer with silicon provides a reverseeffect, thereby forming a high-resistant boron phosphide layer.

A p-type electrode is provided on the surface of the p-type uppercladding layer. The p-type electrode is preferably fabricated from abottom-side electrode and a p-type Ohmic electrode. The bottom-sideelectrode is in contact with the surface of the p-type upper claddinglayer.

As the p-type upper cladding layer has been already endowed with a lowresistance in an as-grown state, the device operation current isprovided restrictively to a light-emitting layer which is a portiondirectly below the upper cladding layer. In order to avoidshort-circuit-like flow of device operation current, the bottom-sideelectrode, provided on the surface of the upper cladding layer so as toattain contact with the layer, is formed of a non-Ohmic material notforming an Ohmic contact with the p-type boron phosphide-basedsemiconductor serving as the upper cladding layer. Examples of preferredmaterials for forming the bottom-side electrode included in the p-typeelectrode include alloys containing a Group IV element such as agold-tin (Au-Sn) alloy and a gold-silicon (Au—Si) alloy. Tin (Sn) has anatomic radius greater than that of boron (B) and phosphorus (P) formingboron phosphide. Therefore, thermal diffusion into the p-type uppercladding layer formed of a p-type boron phosphide-based semiconductorlayer, which may otherwise occur uselessly, can be prevented uponalloying or a similar process, thereby effectively maintaining excellentcrystallinity of the p-type upper cladding layer formed of a p-typeboron phosphide-based semiconductor layer.

Since gold-silicon (Au—Si) alloy contains an element Si, which is moredifficult to diffuse in the boron phosphide-based semiconductor, therecan be effectively prevented an increase in the degree of disorder ofthe p-type boron phosphide-based semiconductor crystalline layer causedby thermal diffusion of silicon. However, film formation of gold-siliconalloy by means of, for example, vapor phase deposition means requireshigher temperature as compared with gold-tin alloy. Therefore, gold-tinalloy is more preferred for forming a bottom-side electrode which canmore effectively prevent thermal deterioration of the p-type boronphosphide-based semiconductor layer, particularly the light-emittinglayer. For the purpose of preventing an increase in the degree ofdisorder of crystal conditions of the p-type boron phosphide-basedsemiconductor crystalline layer, particularly those of thelight-emitting layer, caused by thermal diffusion of elements, thebottom-side electrode is preferably formed of gold-silicon alloy film.Regardless of which of the two alloy films is used, the bottom electrodeincluded in the p-type electrode can be formed of a non-Ohmic material,thereby preventing short-circuit-like flow of the device operationalcurrent via the low-resistance p-type upper cladding layer to a portionof the light-emitting layer directly below the upper cladding layer.

Examples of a material forming the bottom-side electrode includetransition metals such as nickel (Ni), titanium (Ti), and vanadium (V).Among them, titanium (Ti) is particularly preferably used for forming abottom-side electrode, since Ti provides strong adhesion and a highSchottky barrier height to the p-type boron phosphide layer serving asthe p-type upper cladding layer.

When the Ohmic electrode formed of a material able to form an Ohmiccontact with p-type boron phosphide-based semiconductor is provided soas to attain electric contact with the bottom-side electrode, the deviceoperation current, of which flow is inhibited by the bottom-sideelectrode, can effectively be diffused over a wide area of the p-typeupper cladding layer.

In an LED from which emitted light is extracted via the p-type claddinglayer to outside, the light emitted from a portion of the light-emittinglayer corresponding to the projection area of the p-type electrode isdifficult to effectively extract to the outside, because the portion iscovered with the p-type electrode. However, when an Ohmic electrodeformed of a material able to form an Ohmic contact with the p-type uppercladding layer is formed on the bottom-side electrode, flow of thedevice operational current can be provided over a wide area of thelight-emitting layer other than the projection area of the p-typeelectrode. The electrode attaining Ohmic contact with the p-type boronphosphide-based semiconductor can be formed of, for example, an alloycontaining a Group II element such as a gold-beryllium (Au—Be) alloy ora gold-zinc (Au—Zn) alloy. In particular, a gold-beryllium alloy iscapable of forming an Ohmic electrode which has excellent adhesion tothe bottom-side electrode and low contact resistance. Such a p-typeelectrode including a bottom-side electrode formed of a non-Ohmicmaterial covered with an Ohmic material allows the device operationalcurrent to flow in an uncovered portion of the light emission area otherthan the projection area of the p-type electrode; i.e., in a portion ofthe light emission area exposed to the outside, the flow of the deviceoperational current being inhibited by the bottom-side electrode formedof a non-Ohmic material having a high contact resistance with respect tothe p-type boron phosphide-based semiconductor. In order to provide thedevice operation current homogeneously over a wide area of the portionof the light emission area exposed to the outside, the p-type Ohmicelectrode is desirably disposed in terms of shape and intervals suchthat uniform electric potential distribution is provided in a portion ofthe light emission area open to the outside. Such means for disposingthe p-type Ohmic electrode can provide a high-emission-intensity LEDwhich emits light from the light emission area with homogeneousintensity.

The Ohmic electrode is preferably provided so as to extend on a surfacearea of the p-type upper cladding layer other than the bottom-sideelectrode-provided area and to attain contact with the surface area. Forexample, the Ohmic electrode is formed of a stripe electrode extendingon the plane of the light-emitting device symmetrically with respect tothe center. Alternatively, the Ohmic electrode may be formed of aring-shape electrode which is concentric with the center of the planeand in electric contact with each other. These materials can beprocessed to an Ohmic electrode of a desired shape through patterning orselective etching based on a known photolithographic method.

The Schottky contact (non-Ohmic contact) function of the aforementionedbottom-side electrode can be maintained by providing an intermediatelayer formed of a transition metal or platinum (Pt) between thebottom-side electrode and the Ohmic electrode. The intermediate layerformed of a transition metal prevents diffusing or migration of materialcomponents forming the Ohmic electrode to the bottom-side electrode,thereby maintaining the function of the bottom-side Schottky contactelectrode. The intermediate layer is preferably formed of molybdenum(Mo), nickel (Ni), or platinum (Pt), which element can most effectivelyprevent intermetallic diffusion of electrode-forming components betweenthe bottom-side electrode and the Ohmic electrode. The transition metalintermediate layer suitably has a thickness of 5 nm to 200 nm. When theintermediate layer has a thickness as small as less than 5 nm,intermetallic diffusion of the electrode components cannot be preventedto a satisfactory degree, and the bottom-side electrode may become anon-Schottky contact electrode; e.g., an Ohmic contact electrode. Whenthe intermediate layer has a thickness in excess of 200 nm, the distancebetween the Ohmic electrode and the p-type upper cladding layer, whichare in contact with the intermediate layer, increases. Thus, spacebetween the Ohmic electrode and the p-type upper cladding layer may beprovided around the bottom side electrode, thereby disadvantageouslyincreasing the input resistance to the device operation current.

In a boron phosphide-based semiconductor light-emitting device includinga light-emitting layer formed of an n-type Group III nitridesemiconductor; an upper cladding layer which is provided on thelight-emitting layer and which is formed of a p-type boronphosphide-based semiconductor; and a p-type Ohmic electrode formed so asto attain contact with the p-type upper cladding layer, a boronphosphide-based semiconductor amorphous layer that is provided so as toattain contact with the light-emitting layer formed of an n-type GroupIII nitride semiconductor prevents thermal deterioration of alight-emitting layer.

The boron phosphide-based semiconductor amorphous layer that is providedso as to attain contact with the light-emitting layer formed of ann-type Group III nitride semiconductor prevents propagation ofdislocations from the light-emitting layer to the upper layer.

A second amorphous layer which is formed of a boron phosphide-basedsemiconductor vapor-phase grown at a temperature higher than thetemperature at which the aforementioned first amorphous layer serves asan underlying layer for providing a low-resistance upper cladding layerformed of a p-type boron phosphide-based semiconductor in an as-grownstate.

In a p-type electrode provided on the p-type upper cladding layer formedof a p-type boron phosphide-based semiconductor, a bottom-side electrodeformed of a material able to form non-Ohmic contact with a p-type boronphosphide-based semiconductor forming the p-type upper cladding layerserves as a resistor upon passage of device operation current andprevents short-circuit-like flow of the device operation current to anarea of the light-emitting layer corresponding to the projection area ofthe p-type electrode, from which the emitted light is difficult toextract to the outside.

A p-type Ohmic electrode which constitutes the p-type electrode with theaforementioned bottom-side electrode and which is in Ohmic contact withthe p-type boron phosphide-based semiconductor causes the deviceoperation current to preferentially flow to the open-to-the-outsideportion of the light-emitting area.

The first amorphous layer formed of a boron phosphide-basedsemiconductor serves as an underlying layer for providing “adsorptionsites” for promoting vapor phase growth of the second amorphous layer,when the second amorphous layer formed of a boron phosphide-basedsemiconductor is provided so as to attain joining to the first amorphouslayer. For example, the first amorphous layer provides a secondamorphous layer which provides excellent adhesion to the light-emittinglayer. The first and the second amorphous layers formed of an undopedboron phosphide-based semiconductor prevent, for example, inversion ofthe conduction type of the light-emitting layer caused by diffusion andmigration of impurities.

The second amorphous layer which is stoichiometrically rich in boronwith respect to phosphorus imparts the non-stoichiometric composition tothe polycrystalline boron phosphide layer serving as the upper claddinglayer, thereby causing the polycrystalline boron phosphide layer to besuitable for providing the p-type upper cladding layer.

The p-type electrode, which is provided on the p-type upper claddinglayer formed of a boron phosphide-based semiconductor so as to attaincontact with the cladding layer and which includes a bottom-sideelectrode formed of a non-Ohmic contact material, prevents flow ofdevice operation current to a region directly below the electrode andsupplies the device operation current preferentially to thelight-emitting layer such that emitted light can be readily extracted tothe outside. In particular, the Ohmic electrode in electrical contactwith the surface of the p-type upper cladding layer and extending so asto achieve contact also with the surface of the upper cladding layerdiffuses the device operation current, via the p-type upper claddinglayer, over a wide area of the light-emitting layer.

EXAMPLE 1

A double-hetero-junction light-emitting diode (LED) having a pn-junctionstructure including an upper cladding layer formed of p-type boronphosphide and a light-emitting layer formed of n-type gallium nitridewas produced as an exemplary boron phosphide-based semiconductorlight-emitting device. FIG. 2 schematically shows a cross-section of theproduced LED.

A phosphorus (P)-doped n-type silicon {111} single crystal was used as asingle-crystal substrate 101. Firstly, on the {111} surface of thesingle-crystal substrate 101, a lower cladding layer 102 formed of anundoped n-type boron phosphide layer was vapor-phase grown throughatmospheric-pressure (near atmospheric pressure) MOCVD at 925° C. by useof a triethylborane ((C₂H₅)₃B)/phosphine (PH₃)/hydrogen (H₂) system. Then-type boron phosphide layer serving as the lower cladding layer 102 wasformed at a concentration ratio regarding sources fed to a vapor phasegrowth zone; i.e., a V/III ratio (═PH₃/(C₂H₅)₃B ratio) of about 1.3×10³.The thickness of the layer and the carrier concentration were controlledto 300 nm and 1×10¹⁹ cm⁻³, respectively. The thus-formed lower claddinglayer 102 was found to have a forbidden band gap of about 3 eV at roomtemperature.

Subsequently, on the lower cladding layer 102, a light-emitting layer103 having a thickness of 10 nm was formed through atmospheric-pressure(near atmospheric pressure) MOCVD at 850° C. by use of atrimethylgallium ((CH₃)₃Ga)/ammonia (NH₃)/hydrogen (H₂) system. Thelight-emitting layer 103 had a multi-phase structure which was formedfrom a plurality of n-type gallium indium nitride crystalline phases(Ga_(x)In_(1-x)N) having indium (In) composition (1-x) that differ fromone another. The formed light-emitting layer 103 was found to have anaverage indium composition (1-x) of 0.12 (12%) as determined on thebasis of elemental quantitative analysis by means of a transmissionelectron microscope (TEM).

Feeding of trimethylgallium into the vapor phase growth zone was stoppedso as to complete vapor phase growth of the light-emitting layer 103.Thereafter, the temperature of the single-crystal substrate 101 waslowered to 450° C. in an atmosphere containing ammonia (NH₃) andhydrogen. Subsequently, on the light-emitting layer 103, a firstamorphous layer 104 formed of an undoped boron phosphide layer wasformed through atmospheric-pressure MOCVD at 450° C. by use of a(C₂H₅)₃B/PH₃/H₂ system. The first amorphous layer 104 was vapor-phasegrown at a V/III ratio (═PH₃/(C₂H₅)₃B) of 10 so as to cause the layer tobe stoichiometrically rich in boron. The thickness of the layer wascontrolled to 15 nm.

Feeding of (C₂H₅)₃B into the vapor phase growth zone was stopped so asto complete vapor phase growth of the first amorphous layer 104.Thereafter, the temperature of the single-crystal substrate 101 waselevated to 1,025° C. in the vapor phase growth zone where a flow of PH₃and H₂ was maintained.

Subsequently, on the first amorphous layer 104, a second amorphous layer105 constituted by an undoped boron phosphide layer was formed throughatmospheric-pressure MOCVD at 1,025° C. by use of a (C₂H₅)₃B/PH₃/H₂system. The second amorphous layer 105 was vapor-phase grown at a V/IIIratio of 15 so as to form a p-type-conductive layer stoichiometricallyrich in boron with respect to phosphorus. The thickness of the layer wascontrolled to 10 nm.

After completion of vapor phase growth of the second amorphous layer105, the amount of PH₃ fed to the vapor phase growth zone wasselectively elevated such that the V/III ratio was controlled to 1,290,while the amount of (C₂H₅)₃B fed remained constant. Subsequently, on thesecond amorphous layer 105, an upper cladding layer 106 constituted byan undoped p-type boron phosphide crystal layer was formed throughatmospheric-pressure MOCVD at 1,025° C. by use of a (C₂H₅)₃B/PH₃/H₂system. The thickness of the upper cladding layer 106 was controlled to600 nm. The thus-formed upper cladding layer 106 becamestoichiometrically rich in boron, because the vapor phase growth wascarried out at a temperature higher than 1,000° C. and at a low V/IIIratio as described above. The layer was found to have a carrierconcentration of 2×10¹⁹ cm⁻³ and a resistivity of 5×10⁻² Ω·cm, asmeasured through conventional Hall effect measurement performed at roomtemperature. Thus, a low-resistive upper cladding layer (p-type boronphosphide crystal layer) was produced.

Feeding of (C₂H₅)₃B into the vapor phase growth zone was stopped so asto complete vapor phase growth of the upper cladding layer 106.Thereafter, the temperature of the single-crystal substrate 101 waslowered to about 600° C. while the substrate was placed under the flowof a mixture of PH₃ and H₂.

As described above, the lower cladding layer 102, the light-emittinglayer 103, the first amorphous layer 104, the second amorphous layer105, and the upper cladding layer 106 formed of the p-type boronphosphide crystal layer were sequentially stacked on the single-crystalsubstrate 101, to thereby form a stacked structure 20.

Analysis of the thus-produced stacked structure 20 revealed thatselected-area pattern electron diffraction images patterns of the firstand second amorphous layers 104 and 105 exhibit halo diffractionpatterns, confirming that these layers were amorphous. In contrast, theselected-area diffraction pattern of the upper cladding layer 106 wasfound to exhibit a pattern attributable to a {111}-crystal layer,indicating that the upper cladding layer was formed of a p-type boronphosphide crystal layer.

In a bright-field TEM image of the upper cladding layer 106 formed of ap-type boron phosphide crystal layer, substantially no misfitdislocation was visually observed, although presence of twins orstacking faults was identified in a direction parallel to the<111>-crystal direction.

In addition, the ratio of intensity of boron (B) ion to that ofphosphorus (P) ion as determined in field-emission AES analysis revealedthat the boron atom concentration of the first and the second amorphouslayers 104 and 105 and the upper cladding layer 106 formed of a p-typeboron phosphide crystal layer was about 0.5% excess with respect to thephosphorus atom concentration.

Next, the single-crystal substrate 101 on which the stacked structure 20had been formed was cooled to about room temperature and was removedfrom the vapor phase growth zone. Subsequently, a p-type Ohmic electrode107 with a circular shape and formed of gold-beryllium (Au 99% byweight, Be 1% by weight) alloy was placed generally on a center of thesurface of the stacked structure 20; i.e., the upper cladding layer 106formed of the p-type boron phosphide crystal layer. On the entirebackside of the single-crystal substrate 101, an n-type Ohmic electrode108 formed of aluminum-antimony (Al—Sb) alloy was formed. Thus, apn-junction DH structure LED with a square shape having a side length ofapproximately 300 μm was produced.

Upon flow of forward current of 20 mA between the p-type and the n-typeOhmic electrodes 107 and 108, the following emission characteristics ofthe LED were obtained.

-   (1) Color of emitted light: bluish purple-   (2) Center emission wavelength: about 440 nm-   (3) Luminous intensity (as chip): about 6 mcd-   (4) Forward voltage: about 3.5 V

The reverse voltage upon passage of reverse current of 10 μA between thep-type and n-type Ohmic electrodes 107 and 108 was found to be 10 V.

In addition, a near field light emission pattern of the LED confirmedthat the light emission was provided from generally the whole surface ofthe light-emitting layer 103. The reason for provision of such lightemission is considered as follows. In the Example, the upper claddinglayer 106 was formed of a low-resistive p-type boron phosphide crystallayer. Therefore, operation current could diffuse, via the uppercladding layer 106, into a wide area of the light-emitting layer 103. Inaddition, in the Example, the upper cladding layer 106 formed of ap-type boron phosphide crystal layer was formed on the light-emittinglayer formed of a Group III nitride semiconductor (gallium indiumnitride), with the first and the second amorphous layers 104 and 105intervening between the crystal layer and the light emitting layer.Therefore, an LED showing excellent rectifying characteristics; i.e.,causing few local breakdowns, was provided.

EXAMPLE 2

The boron phosphide-based compound semiconductor device according to thepresent invention will next be described in detail, taking as an examplea light-emitting diode (LED) employing a boron phosphide (BP) amorphouslayer, which is a typical boron phosphide-based semiconductor.

FIG. 3 schematically shows the cross-section of a stacked structure 13employed for fabricating an LED 12 having a pn-junction typedouble-hetero (DH) structure. FIG. 4 is a schematic plane view of theLED shown in FIG. 3.

A (0001)-sapphire (α-Al₂O₃ single crystal) was used as a single-crystalsubstrate 101. On the (0001)-surface of the single-crystal substrate101, a lower cladding layer 102 formed of n-type gallium nitride (GaN)was deposited through atmospheric pressure (near atmospheric pressure)metal-organic vapor phase epitaxy (MOVPE) means. The lower claddinglayer 102 was deposited at 1,050° C. by use of a trimethylgallium(molecular formula: (CH₃)₃Ga) as a gallium (Ga) source and ammonia(molecular formula: NH₃) as a nitrogen source. The carrier concentrationof the n-type GaN layer serving as the lower cladding layer 102 wascontrolled to 4×10¹⁸ cm⁻³ through doping with silicon (Si), and thethickness was controlled to 2,800 nm. Feeding of the aforementionedgallium source was stopped so as to complete growth of the lowercladding layer 102. Thereafter, the temperature of the single-crystalsubstrate 101 was lowered to 750° C. in an atmosphere containing anitrogen source (═NH₃) and hydrogen.

Subsequently, on the n-type lower cladding layer 102, a well layer 103a-1 formed of n-type gallium indium nitride (Ga_(0.90)In_(0.10)N) wasformed by use of trimethylindium (molecular formula: (CH₃)₃In) as anindium (In) source and the aforementioned gallium source. The galliumindium nitride layer serving as the well layer 103 a-1 had a multi-phasestructure which was formed from a plurality of phases having indiumcompositional proportions that differ from one another. The averagecomposition of indium was found to be 0.10 (=10%). The thickness of thewell layer 103 a-1 was controlled to 10 nm. On the well layer 103 a-1, abarrier layer 103 b-1 formed of n-type gallium nitride (GaN) wasprovided through the aforementioned atmospheric pressure MOCVD means at750° C. by use of (CH₃)₃Ga)/NH₃/H₂ reaction system so as to attainjoining to the well layer. The thickness of the barrier layer 103 b-1was controlled to 20 nm. Another well layer 103 a-2 formed of theaforementioned Ga_(0.90)In_(0.10)N having a multi-phase structure wasprovided on the barrier layer 103 b-1. The thickness of the well layer103 a-2 was controlled to 8 nm, which is smaller than that of the welllayer 103 a-1, in order to form a band structure in which a conductionband and a valence band are bent, the band structure being advantageousfor emitting long-wavelength light, when the well layer 103 a-2 isjoined to a barrier layer 103 b-2 serving as the final end layer of thelight-emitting layer 103 of the quantum well structure. Subsequently,the barrier layer 103 b-2 serving as the final end layer of thelight-emitting layer of the quantum well structure was provided so as toattain joining to the well layer 103 a-2. The thickness of the barrierlayer 103 b-2 was controlled to 20 nm, which was identical to that ofthe barrier layer 103 b-1.

After the light-emitting layer 103 of the quantum well structure hadbeen formed by alternatively stacking a well layer and a barrier layerand performing the stacking twice, the temperature of the single-crystalsubstrate 101 was lowered to 450° C. in an atmosphere containing anitrogen source (═NH₃) and hydrogen. Subsequently, a first amorphouslayer 104 formed of undoped boron phosphide (BP) was provided so as toattain joining to the barrier layer 103 b-2 serving as the final endlayer of the light-emitting layer 103 of the quantum well structure. Theamorphous layer was vapor-phase grown at a temperature lower than thebarrier layer growth temperature and the well layer growth temperature.The first amorphous layer 104 formed of boron phosphide (BP) wasprovided through atmospheric-pressure MOCVD means employing atriethylborane (molecular formula: (C₂H₅)₃B)/phosphine (molecularformula: PH₃)/H₂ reaction system. The thickness of the first amorphouslayer 104 was controlled to 15 nm. After formation of the firstamorphous layer 104 was complete, the temperature of the single-crystalsubstrate 101 was elevated from 450° C. to 1,025° C. in an atmospherecontaining a phosphorus source (═PH₃) and hydrogen.

Subsequently, through the same atmospheric-pressure MOCVD meansemploying a (C₂H₅)₃B/PH₃/H₂ reaction system and by use of the same vaporphase growth apparatus as described above, a second amorphous layer 105was provided so as to attain joining to the first amorphous layer 104 ata temperature higher than the first amorphous layer 104 growthtemperature. As the second amorphous layer 105 was vapor-phase grown ata V/III ratio (═PH₃/(C₂H₅)₃B) of 16, the second amorphous layer 105assumed a p-type conduction layer which was stoichiometrically rich inboron (B) with respect to phosphorus (P). The thickness of the secondamorphous layer 105 which had been vapor-phase grown at high temperaturewas controlled to 15 nm.

Subsequently, through the same atmospheric-pressure MOCVD meansemploying a (C₂H₅)₃B/PH₃/H₂ reaction system and by use of the same vaporphase growth apparatus, an upper cladding layer 106, formed of anundoped p-type boron phosphide single-crystal layer, was provided so asto attain joining to the second amorphous layer 105 at 1,025° C. Thethickness of the undoped p-type boron phosphide single-crystal layerserving as the upper cladding layer 106 was controlled to 580 nm.

After the stacked structure 13 had been formed through the final vaporphase growth of the upper cladding layer 106, the stacked structure 13was cooled to room temperature. Thereafter, the p-type upper claddinglayer 106 and the light-emitting layer 103 were evaluated electricallyand crystallographically. The p-type upper cladding layer 106 was foundto have carrier concentration of 2×10¹⁹ cm⁻³, as measured through aconventional electrolytic C-V (capacitance-voltage) method. The valueindicated that the upper cladding layer assumed a low-resistive p-typeconduction layer in an as-grown state. The mean dislocation density ofthe layer, as determined through a conventional cross-sectional TEMtechnique, was found to be less than 1×10³/cm², with the presence of aportion having a dislocation density of 1×10²/cm² or less. The barrierlayers 103 b-1 and 103 b-2 and the well layers 103 a-1 and 103 a-2forming the light-emitting layer 103 were found to have an internaldislocation density of about 2×10¹⁰ cm⁻². The thickness of each of thebarrier layers and the well layers forming the light-emitting layer ofthe quantum well structure remained unchanged. Notably, no voidsresulting from decomposition of GaN at high temperature was observedinside the barrier layer serving as the final end layer formingheterojunction with the amorphous layer. In particular, according to theExample, the first amorphous layer joined to the barrier layer servingas the final end layer of the quantum well structure, the secondamorphous layer grown at higher temperature so as to attain joining tothe first amorphous layer, and the upper cladding layer formed of p-typeboron phosphide layer formed on the second amorphous layer serving as anunderlying layer, were all formed from an undoped layer. Therefore, anincrease in the degree of disorder in the junction interface between abarrier layer and a well layer due to diffusion of added impurities wasfound to be reduced.

A p-type electrode 204 was provided on a center of the p-type uppercladding layer 106 serving as the surface of the stacked structure 13. Abottom-side electrode 204 a included in the p-type electrode 204 wasformed of a gold-tin alloy (Au: 98% by weight, Sn: 2% by weight),forming non-Ohmic contact with a p-type boron phosphide single crystal.The bottom-side electrode 204 a had a circular plane shape with adiameter of 130 μm. On the bottom-side electrode 204 a, a p-type Ohmicelectrode 204 b formed of a gold-beryllium alloy (Au: 99% by weight, Be:1% by weight) was provided. As shown in FIG. 4, the p-type Ohmicelectrode 204 b was formed of two stripe electrodes (width: 60 μm)perpendicular to each other. The crossing point defined by two stripeelectrodes 204b perpendicular to each other and the center of the planeof the bottom-side electrode 204 a were caused to be matched. Inaddition, the stripe electrodes 204 b were caused to extend on a portionof a light emission area 205 of the LED 12 which was open to theoutside. The bottom-side electrode 204 a and the stripe electrodes 204 bdisposed on the bottom-side electrode 204 a were coated withvacuum-vapor-deposited gold (Au) film 204 c (thickness: about 1.7 μm),so as to provide a pad electrode for performing wire bonding. On theother hand, as shown in FIG. 4, an n-type electrode 108 wasplasma-etched by use of a gas mixture of methane (molecular formula:CH₄)/argon (symbol of element: Ar)/H₂, whereby an unnecessary portionwas removed to expose a potion of the lower cladding layer 102. Then-type electrode 108 was disposed on the exposed portion of the lowercladding layer 102 as shown in FIGS. 3 and 4.

Emission characteristics of the LED 12 which was equipped with thep-type electrode 204 of the aforementioned structure and which had aplane shape of a square having a side length of 300 μm were confirmedupon passage of device operation current in the forward direction. TheLED 13 emitted blue light having an emission center wavelength of 442nm, with a half-width value observed in the emission spectrum of 120meV. Luminous intensity of the LED chip before being resin-molded, asdetermined through a conventional photometric sphere, was 7 mcd.Luminescent microspots, which have been conventionally generated throughshort-circuit-like flow of device operation current to thelight-emitting layer 103 of the quantum well structure via dislocations,were not observed, because the p-type electrode 204 was provided so asto attain contact with the upper cladding layer 106 formed of p-typeboron phosphide having a particularly low dislocation density. A nearfield light emission pattern indicated that emission intensity wasuniform on virtually the entire portion of the surface of the emissionarea 205, which was open to the outside.

Furthermore, as the p-type electrode 204 was provided so as to attaincontact with the upper cladding layer 106 formed of p-type boronphosphide having a low dislocation density, no local breakdown wasobserved. Thus, an LED 12 having excellent rectifying characteristics;i.e., the forward voltage (i.e., Vf) at a forward current of 20 mA wasabout 3 V and the reverse voltage (Vr) at a reverse current of 10 μA wasabout 8 V or more, was provided.

EXAMPLE 3

FIG. 5 is a schematic cross-sectional view of an LED 30 mentioned inthis Example. FIG. 6 is a schematic plane view of the LED 30. Thecross-section of FIG. 5 is a cross-section along the broken line A-A′shown in FIG. 6. A phosphorus (P)-doped n-type (111)-Si single crystalwas used as a single-crystal substrate 301. Firstly, on the(111)-surface of the single-crystal substrate 301, a lower claddinglayer 302 formed of undoped n-type monomeric boron phosphide (BP) wasdeposited through atmospheric pressure (near atmospheric pressure)metal-organic chemical vapor deposition (MOCVD) means. The n-type lowercladding layer 302 was formed at 950° C. by use of a triethylborane(molecular formula: (C₂H₅)₃B)/phosphine (molecular formula:PH₃)/hydrogen (H₂) reaction system. The thickness of the n-type lowercladding layer 302 was controlled to 240 nm so as to attain areflectance of 40% or higher within a blue light wavelength range of 430nm to 460 nm. Feeding of the aforementioned boron source was stopped soas to complete vapor phase growth of the n-type lower cladding layer302. Thereafter, the temperature of the Si single-crystal substrate 301was lowered to 825° C. in an atmosphere containing phosphine (PH₃) andhydrogen (H₂).

Subsequently, an n-type gallium indium nitride (Ga_(x)In_(1-x)N: 0≦x≦1)layer serving as a light-emitting layer 303 was provided throughatmospheric pressure MOCVD means by use of a trimethylgallium (molecularformula: (CH₃)₃Ga)/trimethylindium (molecular formula: (CH₃)₃In)/ammonia(molecular formula: NH₃)/H₂ reaction system so as to attain joining tothe n-type lower cladding layer 302. The gallium indium nitride layerserving as the n-type light-emitting layer 303 was formed from aGa_(x)In_(1-x)N layer having a multi-phase structure consisting of aplurality of phases having indium compositional proportions (=1-x) thatdiffer from one another. The average composition of indium was found tobe 0.06 (=6%). The thickness of the light-emitting layer 303 formed ofn-type Ga_(0.94)In_(0.06)N layer was controlled to 50 nm. Vapor phasegrowth of the n-type Ga_(0.94)In_(0.06)N layer was completed by stoppingthe feed of (CH₃)₃Ga and (CH₃)₃In.

Thereafter, the temperature of the single-crystal substrate 301 waselevated to 1,000° C. in an atmosphere containing NH₃ and H₂.Subsequently, on the light-emitting layer 303, a first amorphous layer304 formed of an undoped boron phosphide layer was formed throughatmospheric pressure MOCVD means by use of the aforementioned(C₂H₅)₃B/PH₃/H₂ reaction system. The first amorphous layer 304 wasvapor-phase grown at a V/III ratio (═PH₃/(C₂H₅)₃B) of 40. The boronphosphide amorphous layer vapor-phase grown under the conditions wasfound to have an acceptor concentration of 6×10¹⁸ cm⁻³, as measuredthrough a conventional electrolytic C (capacitance)-V (voltage) method.The carrier (hole) concentration at room temperature, as measuredthrough a conventional Hall effect method, was found to be of 4×10¹⁷cm⁻³. The thickness of the first amorphous layer 304 was controlled to12 nm.

Feeding of (C₂H₅)₃B employed as a boron source was stopped so as tocomplete vapor phase growth of the first amorphous layer 304.Thereafter, the temperature of the single-crystal substrate 301 waselevated to 1,050° C. in the vapor phase growth zone where flow of PH₃serving as a phosphorus source and H₂ was maintained. Subsequently, onthe first amorphous layer 304, a second amorphous layer 305 formed of anundoped boron phosphide was formed through atmospheric pressure MOCVDmeans by use of the (C₂H₅)₃B/PH₃/H₂ reaction system. The secondamorphous layer 305 was grown at a V/III ratio of 21 so as to attain acarrier (hole) concentration higher than that of the first amorphouslayer 304. The undoped boron phosphide amorphous layer vapor-phase grownunder the conditions was found to have an acceptor concentration of1×10²⁰ cm⁻³, as measured through a conventional electrolytic C(capacitance)-V (voltage) method. The carrier (hole) concentration atroom temperature, as measured through a conventional Hall effect method,was found to be of 2×10¹⁹ cm⁻³. The thickness of the undoped secondamorphous layer 305 was controlled to 12 nm.

Subsequently, on the second amorphous layer 305, an upper cladding layer306 formed of undoped p-type boron phosphide was formed throughatmospheric pressure MOCVD means at 1,025° C. by use of a(C₂H₅)₃B/PH₃/H₂ reaction system. The upper cladding layer 306 formed ofp-type boron phosphide was vapor-phase grown at a V/III ratio of 21,which was higher than the ratio during growth of the first amorphouslayer 304 and which was equal to the ratio during growth of the secondamorphous layer 305. The upper cladding layer formed of undoped p-typeboron phosphide was found to have an acceptor concentration of 2×10²⁰cm⁻³, as measured through a conventional electrolytic C (capacitance)-V(voltage) method. The carrier (hole) concentration at room temperature,as measured through a conventional Hall effect method, was found to beof 3×10¹⁹ cm⁻³. The resistivity of the layer was found to be 5×10⁻²Ω·cm. The thickness of the upper cladding layer 306 formed of p-typeboron phosphide was controlled to 580 nm. Feeding of (C₂H₅)₃B serving asa boron source was stopped so as to complete vapor phase growth of theupper cladding layer 306 formed of undoped p-type boron phosphide.Thereafter, the stacked structure was cooled to about 650° C. in anatmosphere containing PH₃ and H₂, followed by cooling to roomtemperature in hydrogen flow.

After completion of cooling, the boron phosphide amorphous layers 304and 305 and the upper cladding layer 306 formed of p-type boronphosphide were analyzed in terms of crystal structure. The electron-beamdiffraction patterns of the boron phosphide amorphous layers 304 and 305were halos, and clear X-ray diffraction peaks did not appear. In theX-ray diffraction pattern of the upper cladding layer 306 formed ofp-type boron phosphide, a (111)-diffraction peak attributed to the (111)crystal lattice of zincblende-type boron phosphide was observed as amain diffraction peak. In addition, subpeaks such as (311)and(110)-diffraction peaks also appeared. In the electron-beam diffractionpattern, a number of diffraction spots attributed to the 111-crystallattice of boron phosphide appeared on the 111-diffraction ring. Theresults indicated that the upper cladding layer 306 formed of p-typeboron phosphide was formed of a polycrystalline layer. Observation ofthe inside thereof through the cross-sectional TME technique revealedthat the upper cladding layer 306 formed of p-type boron phosphide was apolycrystalline layer formed up of aggregated columnar (111)-crystalsslightly deviating from the <110> crystal direction. The upper claddinglayer 306 formed of p-type boron phosphide was made from polycrystallineup to the surface portions thereof, and included no single-crystallayer. In contrast, the surface portion of the n-type boron phosphidelayer serving as the lower cladding layer 302 assumed a single-crystallayer having a (111)-crystal lattice.

Since the upper cladding layer 306 formed of polycrystalline boronphosphide had a low resistance, a troublesome thermal treatment forproducing a low-resistance layer, which had conventionally beenemployed, was not performed, and a bottom-side electrode 107 a formed oftitanium (Ti) was provided on a center of the surface of the uppercladding layer. Through a conventional electron beam vapor depositionmethod, a titanium (symbol of element: Ti) electrode having a thicknessof 60 nm was formed. On the bottom-side electrode 307 a having the shapeof a circular plane with a diameter of 130 μm, an intermediate layer 307b formed of platinum (symbol of element: Pt) was provided so as toattain contact with the bottom-side electrode. The platinum layer havinga thickness of 30 nm serving as the intermediate layer 307 b was formedthrough the electron beam vapor deposition method employed for formingthe Ti electrode. In addition, a p-type Ohmic electrode 307 c formed ofa gold-beryllium (Au—Be) alloy was provided so as to attain contact withthe intermediate layer 307 b. The p-type Ohmic electrode 307 c wasplaced around the bottom-side electrode 307 a so as to attain contactwith the surface of the upper cladding layer 306 formed ofpolycrystalline p-type boron phosphide. As shown in FIG. 6, on thesurface of the upper cladding layer 306 formed of p-type boronphosphide, the Ohmic electrode 307 c was arranged in a frame-formelectrode 307 c-1 provided at an external edge of the device and astripe form electrode 307 c-2. The line width of the Au—Be electrode forforming the frame electrode 307 c-1 and the stripe electrode 307 c-2 wasadjusted to 60 μm. The p-type electrode 307 had a three-layer structure;i.e., Ti bottom-side electrode 307 a/Pt intermediate layer 307 b/Au—BeOhmic electrode 307 c.

An n-type Ohmic electrode 308 formed of an aluminum-antimony (Al—Sb)alloy was provided on the almost entire backside surface of the n-type(111)-silicon (Si) single-crystal substrate 301. Thus, an LED 30 havinga pn-junction DH (double hetero) structure was fabricated. Upon passageof operation current (20 mA) in the forward direction between the p-typeand n-type Ohmic electrodes 307 and 308, the LED 30 emitted bluishpurple light having a wavelength of about 440 nm. Luminous intensity ofthe LED (as chip) as determined through a conventional photometricsphere was about 8 mcd. The device operation current, whoseshort-circuit-like flow to the light-emitting layer 303 directly belowthe p-type electrode 307 was inhibited through provision of the Schottkycontact bottom-side electrode 307 a, was diffused over the entiresurface of the p-type upper cladding layer 306 via the p-type Ohmicelectrode 307 c. Therefore, light emission having a uniform intensitywas provided from the virtually whole surface of the p-type claddinglayer 306. Particularly, a variation in emission intensity caused bylong-term passage of device operation current was not observed, asstrain applied to the light-emitting layer 303 was relaxed throughprovision of polycrystalline boron phosphide layers. Excellentrectifying characteristics were attained; i.e., the forward voltage(i.e., Vf) at a forward current of 20 mA was about 3 V and the reversevoltage (Vr) at a reverse current of 10 μA was about 8 V or more.

According to the present invention, the p-type upper cladding layer isformed from a low-resistance p-type boron phosphide-based semiconductorlayer, the layer being grown by the mediation of amorphous layers formedof boron phosphide-based semiconductor. Thus, a boron phosphide-basedsemiconductor light-emitting device which emits high-intensity light fora long period of time and which has excellent rectifying characteristicscan be provided. A boron phosphide-based semiconductor light-emittingdevice according to the present invention is useful for a light-emittingdiode and the like.

1. A boron phosphide-based semiconductor light-emitting device, whichdevice includes a light-emitting member having a hetero-junctionstructure in which an n-type lower cladding layer formed of an n-typecompound semiconductor, an n-type light-emitting layer formed of ann-type Group III nitride semiconductor, and a p-type upper claddinglayer provided on the light-emitting layer and formed of a p-type boronphosphide-based semiconductor are sequentially provided on a surface ofa conductive or high-resistive single-crystal substrate and which deviceincludes a p-type electrode provided so as to achieve contact with thep-type upper cladding layer, characterized in that an amorphous layerformed of boron phosphide-based semiconductor is disposed between thep-type upper cladding layer and the n-type light-emitting layer.
 2. Aboron phosphide-based semiconductor light-emitting device according toclaim 1, wherein the amorphous layer has a multilayer structurecomprising a first amorphous layer being in contact with thelight-emitting layer and a second amorphous layer being in contact withthe p-type upper cladding layer and having a carrier concentrationhigher than that of the first amorphous layer.
 3. A boronphosphide-based semiconductor light-emitting device according to claim2, wherein the first amorphous layer is formed of a boronphosphide-based semiconductor grown at a temperature lower than thetemperature at which the light-emitting layer is formed.
 4. A boronphosphide-based semiconductor light-emitting device according to claim2, wherein the first amorphous layer is formed of an undoped boronphosphide and has a thickness of 2 nm to 50 nm.
 5. A boronphosphide-based semiconductor light-emitting device according to claim2, wherein the second amorphous layer is formed of a p-type boronphosphide-based semiconductor grown at a temperature higher than thetemperature at which the first amorphous layer is formed.
 6. A boronphosphide-based semiconductor light emitting device according to claim2, wherein the second amorphous layer is formed of an undoped amorphousp-type boron phosphide having an acceptor concentration at roomtemperature of 2×10¹⁹ cm⁻³ to 4×10^(°)cm⁻³, a carrier concentration atroom temperature of 5×10¹⁸ cm⁻³ to 1×10^(20 cm) ⁻³, and a thickness of 2nm to 450 nm.
 7. A boron phosphide-based semiconductor light-emittingdevice according to claim 1, wherein the p-type upper cladding layer isformed of a p-type boron phosphide-based semiconductor having adislocation density equal to or less than that of the Group III nitridesemiconductor serving as the light-emitting layer.
 8. A boronphosphide-based semiconductor light-emitting device according to claim1, wherein the p-type upper cladding layer is formed of an undopedpolycrystalline p-type boron phosphide having an acceptor concentrationat room temperature of 2×10¹⁹ cm⁻³ to 4×10^(°)cm⁻³, a carrierconcentration at room temperature of 5×10¹⁸ cm³ to 1×10²⁰ cm⁻³, and aresistivity at room temperature of 0.1 Ω·cm or less.
 9. A boronphosphide-based semiconductor light-emitting device according to claim1, wherein the p-type electrode provided on the p-type upper claddinglayer is formed of a bottom-side electrode and a p-type Ohmic electrode;the bottom-side electrode in contact with the surface of the p-typeupper cladding layer and being formed of a material able to formnon-Ohmic contact with the p-type boron phosphide-based semiconductorserving as the p-type upper cladding layer; and the p-type Ohmicelectrode being in electrical contact with the bottom-side electrode,extending so as to achieve contact also with the surface of the p-typeupper cladding layer, and being in Ohmic contact with the p-type boronphosphide-based semiconductor.
 10. A boron phosphide-based semiconductorlight-emitting device according to claim 9, wherein the p-type Ohmicelectrode is provided so as to extend, as a stripe electrode, on aportion of the surface of the p-type upper cladding layer where thebottom-side electrode is not provided.
 11. A boron phosphide-basedsemiconductor light-emitting device according to claim 9, wherein thebottom-side electrode is formed of a gold-tin (Au—Sn) alloy or agold-silicon (Au—Si) alloy.
 12. A boron phosphide-based semiconductorlight-emitting device according to claim 9, wherein the bottom-sideelectrode is formed of titanium (Ti).
 13. A boron phosphide-basedsemiconductor light-emitting device according to claim 9, wherein thep-type Ohmic electrode is formed of a gold-beryllium (Au—Be) alloy or agold-zinc (Au—Zn) alloy.
 14. A boron phosphide-based semiconductorlight-emitting device according to claim 9, wherein the p-type Ohmicelectrode is formed of nickel (Ni) or a compound thereof.
 15. A boronphosphide-based semiconductor light-emitting device according to claim9, wherein an intermediate layer formed of a transition metal isprovided between the p-type Ohmic electrode and the bottom-sideelectrode.
 16. A boron phosphide-based semiconductor light-emittingdevice according to claim 15, wherein the intermediate layer is formedof molybdenum (Mo) or platinum (Pt).
 17. A method for producing a boronphosphide-based semiconductor light-emitting device, the methodincluding forming a light-emitting member having a hetero-junctionstructure in which an n-type lower cladding layer composed of an n-typecompound semiconductor, an n-type light-emitting layer composed of ann-type Group III nitride semiconductor, and a p-type upper claddinglayer composed of a p-type boron phosphide-based semiconductor andprovided on the light-emitting layer are sequentially provided on asurface of a conductive or high-resistive single-crystal substrate, andforming a p-type Ohmic electrode so as to achieve contact with thep-type upper cladding layer, characterized in that the method comprisesforming an amorphous layer composed of a boron phosphide-basedsemiconductor on the n-type light-emitting layer through a vapor phasegrowth method, and forming the p-type upper cladding layer composed of ap-type boron phosphide-based semiconductor layer on the amorphous layerthrough a vapor phase growth method.
 18. A method for producing a boronphosphide-based semiconductor light-emitting device, the methodincluding forming a light-emitting member having a hetero-junctionstructure in which an n-type lower cladding layer composed of an n-typecompound semiconductor, an n-type light-emitting layer composed of ann-type Group III nitride semiconductor, and a p-type upper claddinglayer composed of a p-type boron phosphide-based semiconductor andprovided on the light-emitting layer are sequentially provided on asurface of a conductive or high-resistive single-crystal substrate, andforming a p-type Ohmic electrode so as to achieve contact with thep-type upper cladding layer, characterized in that the method comprisesforming a first amorphous layer composed of boron phosphide-basedsemiconductor on the n-type light-emitting layer through a vapor phasegrowth method; forming a second amorphous layer composed of amorphousp-type boron phosphide-based semiconductor having a carrierconcentration higher than that of the first amorphous layer through avapor phase growth method such that the second amorphous layer is joinedto the first amorphous layer; and forming the p-type upper claddinglayer composed of a p-type boron phosphide-based semiconductor layerthrough a vapor phase growth method such that the upper cladding layeris joined to the second amorphous layer.
 19. A method for producing aboron phosphide-based semiconductor light-emitting device according toclaim 18, wherein the first amorphous layer is formed on the n-typelight-emitting layer maintained at a temperature higher than 250° C. andlower than 750° C. through a vapor phase growth method at aconcentration ratio of a boron-containing compound as a boron source toa phosphorus-containing compound as a phosphorus source fed to a vaporphase growth zone (V/III ratio) falling within a range of 0.2 to
 50. 20.A method for producing a boron phosphide-based semiconductorlight-emitting device according to claim 18, wherein the secondamorphous layer is vapor-phase grown on the first amorphous layermaintained at a temperature of 1000° C. to 1250° C. at a V/III ratiohigher than that employed in vapor phase growth of the first amorphouslayer.
 21. A method for producing a boron phosphide-based semiconductorlight-emitting device according to claim 18, wherein the p-type uppercladding layer is vapor-phase grown at a temperature of 750° C. to 1200°C. at a V/III ratio falling within a range of 600 to 2,000.
 22. A methodfor producing a boron phosphide-based semiconductor light-emittingdevice according to claim 18, wherein each of the first amorphous layer,the second amorphous layer, and the p-type upper cladding layer iscomposed of boron phosphide (BP).
 23. A light-emitting diode comprisinga boron phosphide-based semiconductor light-emitting device according toclaim 1.